Apparatus for controlling a process



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Patented May 19, n 1953 APPARATUS FOR CONTROLLIN G A PROCES Ralph E. Clarridge, Rochester, N. Y., assigner to Taylor Instrument y Companies,

Rochester,

N. Y., a corporation of New York Application January 10, 1951, Serial No. 205,262

This invention-relates to auid-operated control system for controlling a process variable such as temperature, pressure, rate oftflow, liquid level and the like, at a predetermined value.

Conventional controllers arranged to provide proportional response, especially if 'they are also arranged to provide a derivative response suppleinenting the proportional response, have been effective at the time of start-up of a process to bring the process medium promptly to the desired or set point value Without -overpeaking this value. However, controllers with these two responses fail to prevent off-set, that is, they are unable to restore the process medium to the set-point value if there is a change in the demandv or load on the process. lConventional controllers arranged to provide reset response have been especially effective in preventing olf-set in the process value, due to load changes, Vbut have the limitation of being completely unable to prevent overpeaking. When a conventional controller is arranged to provide proportional response together with both derivative response and reset response, the off-set can be eliminated but the unavoidable interaction between the reset response and the other responses make the elimination of overpeaking impossible. This inability of a reset controller to start up a process without overpeaking has long' been recognized as aserious limitation in automatic control systems. Consequently, it has long been the established standard practice in thisart, that'on highly exacting control processes subject to load changes, where oli-set of thevariable from the set point can not be tolerated, automatic reset must be provided. In this case automatic start-up without overpeaking`is out of the question'and instead start-up of vthe process must be controlled manually. On the other hand, on a process 'where overpeaking or "overshooting the set point value during start-up, is dangerous or at least harmful of the product, proportional response vor proportional plus yderivative response must be used, but the desirable automatic reset response can not be used lin combination therewith, with the result that all process load changes must be controlled manually.

It. is the purpose of thewpresent invention to provide an improved control system wherein proportional response, derivativeresponse and reset response are used, in such a way that the automatic reset response does not 4dominate the other responses. Thus the control lSystem can automatically compensate for load changes and the yderivative response can ybe used .so that the automatic control of the start-up of a process can be 5 Claims- (Cl. 137-492) effected Without 'overpeaking" at the beginning of the process.

The various features and advantages of the invention will appear from the detailed description and claims when taken with the drawings in which:

Fig. 1 is a diagrammatic showing of a conventional controller which is arranged to provide proportional response Vand also derivative re- SDGHSB;

Figs. 2, 2A and 3 are charts useful in explaining the invention;

Fig. llis a diagrammatic showing of a conventional controller of the motion balance type 'arranged to provide proportional respnose, derivative response 'and reset response;

Figs'. 5 and 6 are charts useful in pointing out the deficiencies in the conventional controller of Fig-v4;

Fig. 'l is a diagrammatic showing of the novel controller of the present invention, the proportional response portion of this controller being of the motion balance type;

Fig. 8 is ya 4diagrammatic showing of another embodiment of the novel controller of this invention, the proportional response portion thereof being of the force balance type; and

Fig. 9 is a chart useful in illustrating the highly desirable controller results realized by using the novel controller of this invention, ysuch as is illustrated in Figs. 'l and 8.

A conventional controller arranged to provide proportional response as well as a conventional controller providing proportional response plus derivative response, and ya conventional controller providing all oi these responses together, are all fully disclosed in the patent to Tate et al. Patent No. 2,361,885, granted October 3l, 1944.. However, for the purpose of discussing the present invention a smplied dagram of a proportional response controller in a temperature control system, is shown in Fig, 1, In this system, a process medium in a tank .P is supplied with heat through a steam pipe 5 under control of la motor diaphragm valve 6, The temperature of the medium is sensed by'a bulb 1 projecting therein. This bulb communieating through a capillary tube With the xed end of a Bourdon spring 9, constitutes a tube system which is lled with a thermosensitive fluid. Als this fluid responds to a change in temperature at the bulb 1, the Bourdon spring correspondingly moves its yfree end and a link l0 connected thereto. Therght end of the link I0 moves a loaillelz with `'respect to a nozzle i3. Link l0 also moves a recording pen I4 with respectto the graduated chart I5, to provide an indication of the process temperature. The upper end of the baille is connected by a link I6 to the upper end of a set pointer I'I pivoted at I I for movement with respect to the chart I5. The setting of the pointer at 'a given temperature value on the chart I is effective through the link I=6 to move the baille to the desired set point position of the 'system. In accordance with the usual practice, a source of compressed air at a given pressure, for

example, twenty pounds per square inch, is sup-V plied through restriction I8 and pipe I9 to` the nozzle I3 where it escapes to the atmosphere de- -pending on the space relationship between the baffle I2 and the nozzle I3. If the process temperature measured by the bulb 'I increases above the set point, the Bourdon spring moves the baffle I2 towards the nozzle thereby tending to increase the back pressure in pipe I9. This tends to close a relay valve 20 which tends to supply its output at a decreased pressure. The output pressure of the relay valve 20 is applied through la feedback pipe ZI to a follow-up bellows 22. In response to the decreased vpressure therein, the bellows 22 tends to move-a rod 23 towards` the right. This rod moves the nozzle away -from the baille lan Iamount depending upon the setting 'of the sensitivity adjustment mechanism S to furnish an output porportional to the pen deviation from the set point. The sensitivity adjusting mechanism vS includes hinged bars 25 and 26 which are so arranged that 'as a fulcrum `2'I connected to a rod V28 is moved vertically, the sensitivity of the controller can be changed as indicated by a graduated scale 29. When the temperature at the bulb I drops the controller acts in a similar manner but in the reverseV sense tending to open the control valve to introduce more steam into the process. y

As herein illustrated, ythe feedback pipe 2l has an `adjustable needle valve T therein. When the controller of Fig. 1 is arranged to provide proportional response only, this valve is opened wide so that it has no effect on the operation of the system. However, when this `controller is adjusted to provide la proportional response plus la derivative response, the needle valve is manipulated to restrict the flow through the pipe 2| into bellows 22.. This needle valve together with the capacity of the follow-up bellows '22 yor with a capacity (not shown) supplementing that of the bellows, if desired, provides the derivative response.

The proportional controller For simplicity of rdescription assume that the heat input to the process through pipe 5 is proportional to the opening of valve E. Also assume that the process is operating at a 50% load condition so that the valve '6 will remain one-half open to maintain the process temperature at the set point. To explain the action of the various controller responses, a proportional band will be shown in relation to the process temperature record.y Since sensitivity is control valve movement per pen movement, it follows that the pen of a controller set at a high sensitivityv would have to move but a small amount for full valve travel. Conversely, at la low sensitivity, the pen would have to Ymove through a much greater distance for the full valve travel. Thus the proportional band of la controller is the distance the pen moves to cause the valve to move from its open to its closed position.

When a conventional proportional controller such as illustrated in Fig. 1 (with needle valve 'J2 open) is adjusted to a reasonably narrow band to minimize the effect of load changes, the temperature record at the time of start-up might be very similar to that shown in the chart of Fig. 2. Note that the shaded area, that is the proportional band, extends from the temperature corresponding to the open valve position to that of the closed valve position. In this and subsequent ch'arts vwith the exception of Fig. 9, the position of the temperature graph 35 in the band at any time indicates the opening of the valve. It will be noted in Fig. 2 that `overpeaking occurs especially as indicated at 36 and if this must be avoided, theproportional band can be widened as illustrated in Fig. 2A by lowering the setting of the sensitivity by adjusting the mechanism S, Fig.` 1. While this eliminates the overpeaking,

this adjustment of the controller is often unsatisf-actory because it takes the temperature too long to come in to the set point `and also because the off-set following a load change is greater at low sensitivities, the off-set-being directly proportional to the band width and inversely pro portional to the sensitivity. The equation most often used to express the action of a controller of this kind is change in the controller variable.

S-Constant relating the output tothe input of the controller.

The proportional plus derivative controller If derivative response i-s 'added to the proportional response controllershown in Fig. 1 by restricting the flow through the needle valve T in the feedback pipe 2l thereof, the start-up characteri'stics can be materially improved. The derivative functionhas a stabilizing effect and with a given bandwidth, it reduces the tendency-of the controller to oscillate. For example, invFig. 3 sufficient derivative has been added to prevent over-peaking. If the proportional band is used to showvalve position, the band shifts downward vat the time the process temperature reaches point 31 lat the lower edge of the band. Note that as 'soon as the temperature reaches the proportional band, the derivative response increases the output pressure sufliciently to cause the valve to be approximately 35% open. The amount of shift of the band is ydetermined by the setting of needle valve T. The equation most often used to express this type of performance .as illustrated in Fig. 3, is

the change in the controlled variable. S, lle-Constants relating the output to the input of the controller, the controller adjustments.`

vand a branch pipe 3U to the bellows 24.

The proportional plus reset controller In Fig. 4, there is diagrammatically illustrated a control system that can be arranged to provide proportional response plus reset response or proportional response plus derivative response (sometimes referred to as Pre-Act), plus reset response.

This system differs from that shown in Fig. 1 in that the bellows 22 is opposed by a bellows 24 so that the action of rod 23 in positioning the nozzle I3, is governed by the joint action of this pair of bellows. The output pressure of the relaiT valve is applied through the feedback pipe 2| Reset response is accomplished by restricting the flow through the adjustable needle valve R. in the pipe 30. When the controller is to be used with only proportional plus reset responses, adjustable needle valve T is opened wide so that it has no effect on the operation of the Circuit, since at this time it does not restrict the now through the pipe 2 Il into bellows 22.

When automatic reset response is added to thc proportional response in the controller, the startup temperature record may be very similar to that illustrated in Fig. 5. Note that the proportional band is entirely above the control point during the start-up period and that it does not begin t0 shift downward until the temperature reaches the set point at 38. Due to the fact there is no action until the set point is reached, the overpeaking is considerably worse than it was before theautomatic reset feature was added. However any off-set due to process load changes are eliminated by the use of the reset response. If the instrument sensitivity were lowered with the resultant widening of the proportional band as was done in Fig. 2A, the over-peaking will become worse rather than better. This is due to the fact that the band becomes wider on the upper side of the set point, and therefore, the rising temperature will make a greater excursion into this band before the shift of the band returns the variable to the set point. There has been no satisfactory method suggested for eliminating this over-peaking when the proportional and automatic reset responses alone are used. The equation for this controller is 3) =SR -0 (f-The rate of change of output pressure of the controller.

(at-:e0)-The change in 'the controlled variable.

d x dt S, R-Constants determining the magnitude of the control effects, the controller adjustments.

with the three The rate of change of the controlled variable.

The conventional controller responses the set point is reached. over-peaking can not be eliminated, although it can be reduced.

The equation most often used to describe mathematically, the performance shown in Fig. 6, is

lt will be noted that this equation is obtained by adding the three responses. However, the mechanism herein illustrated and others which have been used in conventional control systems do not produce results which correspond to `this equation.

To illustrate thiareier to Fig. 4 which is a schematic diagram of the pneumatic circuits used in some of the conventional controllers. As the temperature increases, the Bourdon tube moves the baffle l2 towards the nozzle I3, thereby increasing the nozzle back pressure and actuating the relay valve 25J. Since the relay is reverse acting, the output pressure diminishes. If the reset restriction R. is closed (IL- 0) with the output pressure, given when pen and set pointer are together, locked in the bellows chamber 2li and the derivative restriction T is open (T=0) the falling output pressure in the bellows chamber 22 moves the nozzle to the right thereby causing the nozzle to move in a direction to increase the original nozzle-baille spacing due to the change in the variable. If for a given output pressure change in the bellows chamber 22 the nozzle movement is large due to the sensitivity adjustment S, the instrument sensitivity is low and its proportional band is wide. On the other hand, if the nozzle motion for a given output pressure change is reduced by the sensitivity adjustment S, the controller sensitivity is high and the proportional band is narrow. The automatic reset response is added by opening the reset restriction R so that over a period of time, this pneumatic sensitivity reduction is canceled. In other words, the pressure change in the bellows chamber 22 is canceled by a corresponding pressure change in the bellows chamber 24 so that the nozzle is restored to its original position and the temperature is restored to the set point. As the derivative restriction T is gradually closed, the pressure on the control valve 6 must lead the pressure in the bellows chamber 22 by an amount depending upon the restriction. A study of the circuit will show that the magnitude of this lead is dependent upon the rate of change of the controlled variable, but the lead time is primarily a function of the restriction T.

Next, consider the adjustments of this controller S. R and T. First, set R equal to infinity (open the restriction or needle valve) and T equal to zero (open the restriction or needle valve). Since the output pressure enters both the bellows 22 and 24, the nozzle does not move and the controller has a high sensitivity. When R and 'I' yare closed equal amounts (the product of RT equals 1), and again the nozzle does not move and the controller remains in a high sensitivity. While this is known to be the fact from this analysis of the controllercircuits aswell as from actual obserm vation,thisis not indicated by Equation 4. The-refore, Equation 4 does not apply to this particular controller. Without going through the mathematical details, the equation which describes the 7 action of the controller of Fig. 4 with reasonable accuracy is Let this equation be examined to determine how well it nts the observed results. First, set T=J whereupon the equation becomes identical with Equation 4. In other words, the instrument behaves in an orthodox manner with the proportional and automatic reset responses. When the reset restriction R is closed 1-2:0) Equation 5 becomes Equation 2, and again the instrument behaves in an expected fashion. If the product of RT=1,

l-l-RT l-RT becomes innite indicating that the sensitivity of the controller is extremely high. This corresponds with the observed results. Therefore, While it has not been proven that Equation is a good mathematical description of the controller shown in Fig. 4, it appears to correspond with the observed results and this can be verified mathematically.

Now consider the controller to determine Whether its responses can be rearranged to produce more satisfactory results than illustrated in Fig. 6. First, if over-peaking is to be prevented, it is essential that the proportional band must not shift a'bove the control point during the start-up period, as illustrated in Figs, 5 and 6. Since this shifting of the band occurs each time that automatic reset is used in the controller circuits, it is proposed in accordance with the present invention, to separate the pneumatic circuits of the controllerinto two stages with the proportional and the derivative responses in the first stage as illustrated in Fig. 7. Since automatic reset is always necessary to prevent the off-set due to load change, it is, in accordance with the present invention, incorporated in the second stage of the controller circuits so that it will not disturb the proportional band of the rst stage. In as much as the derivative and proportional responses must be transmitted to the diaphragm valve 6, the second stage of the controller circuits must also have a proportional response. In other words, the controller of this invention comprises two stages; the rst incorporating proportional plus derivative responses, and the second incorporating proportional plus automatic reset responses. Since two proportional bands ern'st, this controller can be referred to as a two band controller, the rst band being influenced by the derivative response and the second band, being inuenced by the automatic reset response.

The first stage of the controller of this invention, Fig. 7, has proportional response plus derivative response and its operation is identical with the operation of the controller shown in Fig. 1. Thus for a decrease in the process temperature, the first stage output pressure increases. This output pressure is fed through the pipe 2i to the second stage which has xed proportional response plus reset response. The second stage or reset stage comprises a casing 40 which with parallel diaphragms 4I, 42 and 43 sealed thereto provides a primary chamber 44, a secondary chamber 45 and a tertiary chamber 46. These diaphragms have the effective areas of the proportions indicated. A pillar 41 connected to the centers of these diaphragms partakes of the resultant movement thereof to move in a vertical direction in response to changes in the pressures in the reset stage. The lower end of the pillar 47 carriesva baffle 48 and is biassed upward by a spring 50. The spring 50 is so adjusted that when the reset stage is in its equilibrium condi,- tion the spring will balance the pressure in chamber 44 at the midspan position. The right end of the baffle cooperates with a nozzle 5| to constitute a control couple. A source of compressed air under uniform pressure is supplied through restriction 52 and an output pipe 53 to bleed to the atmosphere through the nozzle 5l under control of the baffle 48. As the baffle changes its position with respect to the nozzle 5|, the output pressure in the pipe `53 will vary accordingly. This pressure is applied to a booster relay.

The booster relay comprises a casing 60 which with parallel diaphragms 8l and 62 sealed thereto, provides chambers 63, 64 and 65. The diaphragms are connected at their centers by a pillar 56 so that they operate as a unit against the biassing action of spring ll'l in response to thediilerence in pressures applied to their respective surfaces. The pillar 66 has a passage 66a therein leading to chamber S4 which communciates with the atmosphere through lan opening 88. Pillar 66 has a valve seat at the entrance to the passage 66a, controlled by a ball valve G9 provided at the upper end of van up wardly spring-biassed valve stem 10 governing the rlow of air through the passage. Compressed air from a suitable source is supplied through the pipe Il into chamber 65 under the control of a ball valve 'I2 carried on the lower end lof the valve stem 1B, and cooperating with a seat formed in the bottom cap at the entrance to chamber 65. The pressure in the chamber 65 is' applied to the diaphragm 62 tending to oppose the pres- Sure applied to diaphragm 6I comprising a part or the chamber B3. Compressed air is supplied from the chamber 55 through the pipe I3 which communicates with the diaphragm motor of valve 6, to position this valve so that the proper amount of steam is supplied through the pipe 5, to correct any deviation from the desired value of the process. The pressure in the pipe 13 is also applied directly to the chamber 45 and is also applied through a delaying connection including the reset needle valve R and capacity 54, to the chamber 46.

To explain the operation of the controller, assume a decrease in the process temperature which actuates the Bourdon tube 9 through the bulb l and capillary 8. The first stage, containing the sensitivity adjustment S and derivative needle valve T. operates as previously described,

to furnish an increase in output pressure in the pipe 2l and the chamber 44. This causes the pillar 41 to move downward thereby bringing the bale 48 closer to nozzle 5l. This relative movement of the baille and the nozzle results in a change in pressure in pipe 53 such that the pressure in the chamber 63 of the booster relay increases. This causes the pillar 65 to move downward tending to move the ball 17.' further from its seat with the result that air at an increased pressure is supplied through the booster relay to pipe 13 leading to the motor diaphragm valve 5. Valve E tends to open thereby increasing the amount of steam supplied to the process. The pressure in pipe 'I3 is also supplied through the branch pipe 14 to chamber 45 tending to increase the nozzle-baule displacement which res ults in pneumatic sensitivity reduction r feed back action. The pressure in the pipe 14 is alsov supplied through the reset valve R; and capacity 54 to chamber it causing the pillar y41 to move downward. This will, as previously explained, cause the output to the control valve ii to increase. Thus through the action of the reset valve R and'chamber lit, the output pressure will .Continue to increase until the pen returns to the set point. Thus any oi-set due to a load change is eliminated. When the process temperature is at the set point, the pressures in chambers 45 and de will be equal, the pressure in chamber ed will be at its midspan value and will be balanced by spring l), and pillar 41 will be at its normal cr equilibrium position. An increase in the process temperature results in a similar operation of the control system but in the reverse sense.

n they form of the invention disclosed in Fig. 7, the rst lstage therein illustrated above the broken line, is of the motion balance type.; Howeverthis invention is not so limited but also includes a control system in which the rst stage is of the force balance type as disclosed in the modified form shown in Fig. 8. This modified form of the invention, by way vof example, cornprises a system for the control of temperature of a given process P to which lsteam is supplied through the pipe 5 under the .control of a diapnragm motor valve 6., the valve being controlled by the system to admit the proper amount of steam into the .process to maintain it at the desired temperature.

The temperature of the process P is sensed by temperature transmitter '80 including a capillary tube 8 of the tube system which terminates in a bulb 'l exposed to the process medium. This transmitter may be similar in construction to that disclosed in the patent application of Mather et ai., Serial No. 190,776, iiled December 9, k1947, now Patent No. 2,536,198, granted January 2, i951. The temperature transmitter et transmits air or the like under pressure from the source pipe 8|, at a signal pressure inversely proportional to the temperature changesensed at the bulb 1. This proportional pressure is communicated through the pipe .82 to the first stage..

The rst stage of the system comprises a casing 33 with parallel diaphragrns 84, 85 and 86 of the effective areas indicated, sealed at their margins to the inner wall of the casing to deiinethe chambers 81, .8B and 89. The centers of the diaphragm vare secured in sealed relation to a pilla-n.90 to move it up .or down depending on the resultant of the forces lon the several diaphragme, caused by -thepressures in the chambers 81, e8 and 89. The pillar 90 carries Lthe left end of baliie 9| which pillar is normally biased upward by a spring 92. The yright end of the baiile cooperates with a nozzle S3 to constitute a control couple. The signal-pressure in the pipe 82 is applied to primary chamber 88and a selected control pressure, which vdetermines the set point or desired temperature of the system, is applied -to the secondary vchamber .89 through the reducing valve V. yA y'feedback pressure from the output of this rst stage, as will be described isapplied to the tertiary chamber 81. Fluid under pressure, such as compressed air under uniform pressure, is supplied through restriction-SM and pipe `96, to rnozzle` where it bleeds tothe atmosphere depending -on ythe kspacing between the baiiie' 9| and nozzle 93." ThisVT action controls the output pressure in pi@ .95 which pressure is fed back through the adjust# able restriction or needle Valve T, to the tere 'tiary chamber 8?, the needle valve 'l' together with the capacity 99, controlling the rate of this pressure feed back to provide proportional response plus derivative response in pipe 91. Y

Pipe 31 COlIllnunicates with an isolating oneto-one relay. vThis relay comprises a casing IIJ!)` which is divided into an upper chamber |0| and lower chamber |92 by a diaphragm w3 which` has its margin sealed in the walls of the casing to prevent duid from passing from one of these chambers to the other. A source of compressed air at uniform pressure is supplied through restriction |65 to the chamber while the controlled pressurel in branch pipe 91 from the output of the first stage, ,iS applied to chamber |02. llille top of the casing is provided with anv in-A wardly directed nozzle |04 exhausting from. chamber l|'|| to the atmosphere, under the control of the diaphragm |03 which cooperates with the nozzle to vary the escape of fluid therethrough. This relay which serves to prevent the second stage from' reacting back on the rst stage, functionsA to deliver t0 its output pipe' |61, compressed air at a pressure equal to that ,in its input pipe 91.

Output pipe |01 communicates with a pressure dividing network whereby the sensitivity of the system can be adjusted at will. This sensitivity adjusting means determines the amount that the valve B opens or closes for any given change in temperature. The network comprises a pipe lli) having the sensitivity or `gain restrictions or needle valves GI and G2 connected therein. The left end of pipe lli) communicates with the out put pipe lll'i while theright end of pipe l 0 communicates with a pipe |09 leading to a separate source oflud under pressure. The pipe |09 has a manually operated pressure reducing valve m3 therein whereby Compressed lair at a desired pressure can be delivered to the network.

The pipe 'H2 which communicates With the pipe l0 at a point between the needle valves Gl and G2 leads tothe chamber d4 of the second stage. It will rbe understood that 'by appropriate adjustments of the needle valves G| and G2 any desired vfraction of the pressure in the pipe |01 is delivered to the chamber dil of the second stage. The remaining portion of the modied system shown in Fig. 8 is identical with the .corresponding portion .shown below between .the dotted line -of Fig. 7 and the description thereof need not be repeated.

vIn the system of Fig. 8 an y.isolating relay has been disclosed. Jcoin-sure that the pressure changes occurring in the pressure dividing'network` will not react back `on the first stage. has been discovered that the Vpipe 91 can communicate directly with the pipe |611 and yet pressure changes in the pressure dividing' network will not vrea` ct vback on the iirst stage if tjhe circuits constants cf the pressure dividing network lare properly selected.

In the' operation of the system of Fig. 81et it be assumed that the ,temperature at bulb l1, 'is stable at the set point, then .the signal pressure in pipe 82 andthe reference or set point pressure are equal, at which time the pressures ,in ,chambers 81, .88 and 89 are equal. yThen assume an increase in process temperature at :bulb '1 which actuates the transmitter 8|) Acausingr it to deliver to the pipe 82 and to chamber .38 a signal pressure inversely proportional to the change in temperature sensed at bulb 1. This decrease in pressure in chamber 88, since it is less than the reference pressure in chamber 89, tends to permit thepillar 9D to move upward thereby bringing baffle 9i closer to the nozzle 93. This movement of the baille with respect to the nozzle, increases the back pressure in pipe 96 and in the branch pipe 91. The pressure in pipe 96 is applied through needle valve T to chamber B1. 'I'he resistance of needle valve T in combination with thecapacity of chamber 81 and the external capacity 98, provides a derivative action whereby the pressure in chamber 8'! is proportional to the rate of change of signal pressure. The action of chamber 81 operating through the balile 9| and nozzle 93, further modifies the pressure in the output pipes 96 and 97 so that this pressure is proportional to the'change in value of temperature at bulb l and is also proportional to the rate of change of this temperature. pipe 91 is communicated through the isolating relay tothe pressure dividing network. This network supplies to the chamber 44 of the second stage a selected fraction of the pressure in the pipe 91. The second stage together with the remainder of Fig. 8 below the broken line functions in the same manner as the corresponding portion of Fig. '7 and need not be further described.

vWhen using the present improved controller as disclosed in Figs. 7 and 8, the temperature record at the time of start-up might be very similar to that shown in the char-t of Fig. 9. Note that each stage, the rst incorporating derivative response and the second incorporating reset response, has a separate and non-interacting band. Thus the good features of each can be utilized. without any resulting disadvantages. Note also that while the band of the second or reset stage is above the set point, the first stage band is divided by the set point. Thus the output pressure of the first stage in the pipe 2| is throttled as soon as the temperature reaches the bottom edge of the rst stage band. Since the first stage includes derivative response the band 'i shifts downward an amount depending upon the setting of needle valve T. The position of the temperature graph 35 within this band is indicative of theoutput pressure of the rst stage of the control system and it can be seen in Fig. 9

The pressure in that vimmediately after the temperature enters 'I the band, the output is more than 50% of its full range. Sincethe output pressure from this portion'of the control circuit now communicated to a relay incorporating reset, this relay is actuated as soon as the pressure reaches 50% of its range and the second stage band begins to shift downward from its previously unfavorable position. As far as the reset action is concerned, the process temperature has apparently reached the set point at the time the output from the rst stage reaches its mid-range pressure. Due to the derivative action, it is possible to make this output pressure reachits mid-range position long before the `temperature reaches the actual set point. Thus the controller can shift the second stage band downward so that overpeaking is prevented. With this type of controller, both overpeaking and off-set can be eliminated and no manual control is necessary on either start-up or on load changes.

From experience with conventional controllers, it would be expected that on difficult applications the throttling range or proportional band would have to be widened excessively in order to prevent overpeaking, and this in turn would allow the temperature to deviate too much with load changes. However, with the controller o'f this invention, the sensitivity can be lowered to a value which seems excessive, but it can `be shown that this new controller will produce exactly the same responses to load changes as the conventional controller would with its sensitivity set at a considerably higher value. Furthermore, there are new values of the derivative time and the reset rate which must be used if the full beneiits of the present control circuit are to be obtained.

Without developing the equation in detail, the controller in Fig. '7 can be described by the following equation:

It will be noted that in Equation 4 S, R, and T have been used to denote the instrument Iadjustments in a non-interacting controller. In Equation 5 R, S, and T have 4been used to denote the controller adjustments in a conventional controller'. In Equation 6 R,'S, and T', are the adjustments in the controller of this invention while n is the gain in the reset'relay, thereof. It should be noted that this gain'factor n multiplies the proportional response in the same way that thesensitivity (S) does.

Ii" the present controller, the conventional controller, and the non-interacting controller are to perform in an identical manner in response to load changes, the coefcients of each term in the equation must be equal.

In other words,

Since we are interestedin the dial readings on the new instrument in terms of the dial readings on the old instrument, rather than in terms of the corresponding readings on the non-interacting controller, the last two sections of Equations '7, 8 and 9 may be solved. In solving, a quadratic equation was obtained which has tvv-o solutions. The rst solution is These equations are Very helpful in adjusting lthe new controller and in illustrating that every satisfactorynadjustment of the conventional controller can be duplicated on the new controller, although the values of the instrument settings may differ considerably.

For example, on a specific problem if the product of the reset rate and preact timelRT) is equal to A when the best possible adjustment of the instrument is obtained, the corresponding settings of the new controller can be obtained from Equations and 11. Specifically, vit will be noted that the product of the apparent sensitivity and the gain (nS) in the new controller must be 1/3 greater than the sensitivity reading on the conventional controller if the solution of Equation 10 is accepted. With this solution the reset rate (R) and the derivative time (T) dial readings are identical with those on the conventional controller. These adjustments will produce a performance Within the control band which is identical with that of the previous controller. However, Equation 11 proposes another solution in the adjustment of sensitivity, reset rate, and derivative time of the new controller which will also produce identical results within the control band. In the case selected when RT=1/4 the product of the apparent sensitivity and the gain in the new controller will be 1A; of the sensitivity ci the conventional controller, while the new reset rate setting is the reciprocal of the former derivative time, and the new derivative time setting is the reciprocal of the former reset rate.

Now the first solution (Equation 10) suggests a narrower proportional band than that which existed with the previous controller. However, due to the fact that this proportional band is not shifted by the automatic reset response, the start-up characteristics with the controller ad- .l'usted in accordance with this solution are better than those of the conventional controller. #The second solution (Equation 11) suggests a much wider proportional band than that which existed with the previous controller. This wide proportional band is extremely helpful during the startup period, since it enables the derivative response to actuate the balance of the controller long before the control point is reached. In other words, the output of the rst stage in the controller has a lead factor with respect to the actual variation of the controlled variable whereby the sensed change in the variable condition effectively appears to take place earlier than it actually did. In this Way overpeaking on start-up can be prevented on even the most difficult applications. Therefore, this second solution clearly indicates that even though our proportional band is Widened considerably in order to obtain a satisfactory performance on start up, the operation of the controller following a load change need not be impaired.

While this invention has been disclosed in a system of the pneumatic (elastic uid) type it is also applicable to hydraulic and to electrical systems. It Will be understood that the present disclosure is given merely by Way of example and that there can be numerous embodiments of the present invention, within the scope of the appended claims, without departing from the spirit of the invention. While an abbreviated operated in response to a signal change for depressure o the delivered fluid and responsive to a stabilizing pressure equal to that of the r'lal pressure,`and means responsive to the supplied fluid for restoring said condition to said predetermined value. f.

2. vIn a system for maintaining a variable condition at a predetermined Value, means for seilsing the value of the variable condition and for producing a feeod. Signal piOpOltlO'Ilal to the sensed value, a primary unitincluding feed backl mechanism operated in response a recorded signal change for delivering elastic fluid under pressure proportional to the magnitude of the signal change and to the rate of change of the signal, a secondary unit including automatic reset mechanism operated responsive to the delivered pressure for supplying elastic fluid under pressure proportional solely to the delivered pressure, to the supplied fluid pressure and to the integral with respect to time of the delivered lluid pressure, and means responsive to the supplied uuid for restoring said condition to said predetermined value.

3. In a system for maintaining a variable condition at a predetermined value, means for sensing the value of the variable condition, means translating the sensed value into a pneumatic signal proportional to the sensed value, a primary unit inclu-ding feedback means operated in response to a signal change for delivering elastic fluid under pressure proportional to the magnitude of the signal change and to the rate of change of the signal, a secondary unit including automatic reset mechanism operated responsive to the delivered pressure for supplying elastic fluid under pressure proportional solely to the delivered pressure and to the integral with respect to time of the change in pressure of the delivered fluid and to a stabilizing pressure equal to that of the final pressure, and means responsive to the supplied uid for restoring said condition to said predetermined value.

4. In a system for maintaining a variable condition at a predetermined value, means for sensing the value of the variable condition, means translating the sensed value into a pneumatic signal proportional to the sensed value, a primary unit including feedback means operated in responsegto a signal change for delivering elastic fluid under pressure proportional to the magnitude of the signal change and to the rate of change of the signal, a secondary unit including automatic reset mechanism operated responsive to the delivered pressure for supplying elastic iluid under pressure proportional solely to the delivered pressure, to the supplied iluid pressure and to the integral with respect to time of the delivered fluid pressure without retrograde influence on said feedback means, and means responsive to the supplied fluid for restoring said condition to said predetermined value.

5. In a system for maintaining a variable condition at a predetermined value, means for sensing deviations in the value of the variable condition and for transmitting a fluid under pressure corresponding to the sensed deviations; a force balance control unit comprising cooperating 15 force responsive members, at least a given one of which is responsive to the pressure of the transmitted fluid, a source of uid under pressure, a valve actuated jointly by said force responsive members for supplying iluid from said source, in part, responsive to the application of the transmitted pressure to said given member, means including said valve and a connection adjustably restricted for applying the supplied fluid to another of said force members in a direction to oppose the force due to the transmitted iluid under pressure whereby the resultant operation of said valve supplies fluid from said source at a pressure proportional to the sensed deviation and proportional to the rate of change of the sensed deviation, a secondary unit including automatic reset mechanism operated responsive to the supplied pressure for delivering elastic iiuid under pressure proportional solely to the supplied pres- 16 sure, to the delivered fluid pressure and to the integral with respect to time of the supplied nuid pressure, and means responsive to the delivered fluid for restoring said condition to said predetermined value.

RALPH E. CLARRIDGE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,361,885 Tate et al Oct. 31, 1944 2,409,871 Krough Oct. 22, 1946 2,512,561 Ziegler June 20, 1950 2,520,468 Moore Aug. 29, 1950 FOREIGN PATENTS Number Country Date Great Britain May 19, 1941 

